Conventional water heaters provide heated water by storing heat energy in the water. Because the heat energy is stored in the water, a large portion of the heater must be dedicated to storing the heated water until it is used. Typically, water stores approximately 10 calories/gram of heat per 10xc2x0 C. increments. By requiring a large portion of the heater for storage of heated water, the water heater must be of reasonable size to store enough heated water for use in a standard residential environment. Typical residential water heaters have a 50-60 gallon capacity. For use in buildings and industry, water heaters must be even larger.
Water heaters consume a high percentage of residential energy heating water for bathing, washing dishes, washing clothes and heating homes and offices. In homes heated by electricity, the consumption of electric power is even greater. Overall, a large imbalance in electric power usage exists during the day time due primarily to the large amounts of power consumed by industry, businesses and public transportation. In many locations, day time peak usage is twice that of the night hours. To compensate for the extensive day time use of electric power, utility companies provide generating capacity sufficient to supply day time usage, leaving unused capacity available for the night hours.
In order to reduce an ever expanding need for day time power, utility companies are providing incentive pricing for off-peak power usage to encourage more use of the existing surplus night power capacity. This practice is common in Japan, in some European countries, and in a few locations in the United States.
Further, the ever increasing use of fossil fuels world wide for heating and cooling applications contributes significantly to the depletion of irreplaceable raw materials. Additionally, the burning of fossil fuels of all kinds is resulting in atmospheric pollution with nitrogen oxides and in higher concentrations of carbon dioxide in the atmosphere. The carbon dioxide buildup is of increasing concern because it may eventually result in drastic changes in global climates through global warming.
Thus, a need has arisen in the art for a water heater which can more efficiently heat water, which can make effective use of utilities during off-peak hours to minimize building and household power consumption and which would consequently reduce building and household utility costs. Further, there is an urgent need to adopt energy conservation technologies to stretch the supply of available fossil fuels until non-polluting energy can be developed and implemented.
The present invention solves the existing needs in the art by providing a water heater which can more effectively heat water, which can make efficient use of electricity during off-peak hours to minimize household power consumption in homes using electricity and which consequently reduces household utility costs. The invention also addresses the need to reduce fossil fuel consumption and the resulting air pollution and carbon dioxide build-up through the use of renewable solar photovoltaic and wind energy to supply all the power for the water heater.
The water heater includes a material, preferably a phase change material, having a thermal energy capacity of at least about 25 cal/g and having a freezing/melting temperature from about 20xc2x0 to about 100xc2x0 C., and more preferably, from about 40xc2x0 C. to about 80xc2x0 C. The water heater further includes a source of water; a heating element for heating the water from the source; a heat exchange unit which contains the material; and a plurality of heat exchange tubes positioned in the heat exchange unit, with the material being located between and around the heat exchange tubes, where the tubes are in heat transfer relation to the phase change material and in fluid connection with the source of water so that the water heated by the heating element flows through the tubes and heats the material. Thereafter, the heat stored in the material is transferred through the tubes to water flowing therethrough at nearly constant temperatures equivalent to the freezing/melting temperature of the material.
Preferably, the water heater further includes a shell having an exterior surface, the heating element and the heat exchange unit being enclosed in the shell, and a layer of insulation on the exterior surface of the shell. The insulation preferably has an xe2x80x9cRxe2x80x9d value of at least about 10 and more preferably comprises vacuum panel insulation having an xe2x80x9cRxe2x80x9d value of at least about 20 per inch of thickness.
In use, the heating element heats water which then heats the phase change material. In one embodiment of the invention, the material is in the form of a powder comprising a phase change material and finely divided silica particles. In this embodiment, the phase change material preferably comprises a crystalline alkyl hydrocarbon. In a preferred embodiment, the material comprises from about 50 to 80% by weight of the phase change material and about 20% to 50% by weight silica particles, and more preferably, about 65% by weight of the phase change material and about 35% by weight silica particles.
In another embodiment of the invention, the material is in the form of a reversible gel which comprises a phase change material and finely divided silica particles. In this embodiment, the phase change material is selected from the group consisting of linear alkyl hydrocarbons, fatty acids, and fatty acid esters. In one embodiment, the gel comprises from about 25 to 70% by weight of a fatty acid phase change material and from about 15 to 30% by weight finely divided silica particles. In another embodiment, the reversible gel comprises from about 78 to 90% by weight of a linear alkyl hydrocarbon phase change material and from about 10 to 22% by weight finely divided silica particles.
In yet another embodiment of the invention, the material used in the water heater is a phase change material comprising a primary alcohol. The phase change material may be in the form of 100% liquid primary alcohol. Alternatively, the phase change material may be in the form of a powder comprising a mixture of a primary alcohol and a silica dry powder. In another embodiment, the phase change material is in the form of a reversible gel comprising a mixture of a primary alcohol and silica particles. In yet another embodiment, the phase change material comprises a solid melt mix of a primary alcohol, high density polyethylene, ethylene-vinyl acetate and silica.
In yet another embodiment of the invention, the material is a phase change material selected from the group consisting of primary alcohols, linear alkyl hydrocarbons, fatty acids, and fatty acid esters. In this embodiment, the material is imbibed into a foam which surrounds the heat exchange tubes of the water heater. Preferably, the foam is an open-celled foam having nano-sized pores.
In still another embodiment of the invention, the material is a salt hydrate phase change material. In this embodiment, the phase change material is selected from the group consisting of sodium sulfate decahydrate, calcium chloride hexahydrate, trisodium phosphate dodecahydrate, and sodium acetate trihydrate.
Accordingly, it is an object of the present invention to provide a water heater which employs a material such as a phase change material to heat water. These, and other objects and advantages of the present invention, will become apparent from the following drawings, detailed description and claims.